Patent documents U.S. Pat. No. 5,214,339, U.S. Pat. No. 5,461,273 and U.S. Pat. No. 5,479,063 disclose, for example, a method and a device for exciting the actuator of an ultrasonic wave motor. In the method described in these cases, the frequency of the electrical voltage of the excited ultrasonic actuator is regulated by maintaining a constant phase difference between the exciter voltage and the voltage that is generated by the auxiliary electrode arranged on the piezo-element of the ultrasonic actuator.
The disadvantage of this method and the corresponding excitation device is that the phase difference between the electrical excitation voltage of the actuator and the voltage of the auxiliary electrode depends on the mechanical load on the actuator. Therefore, under the influence of high mechanical loads on the ultrasonic actuator, in these motors the frequency of the electrical excitation voltage is not equal to the mechanical resonant frequency of the ultrasonic actuator. This has a destabilizing effect on the functioning of the ultrasonic motor. In addition, for safe operation the auxiliary electrode must have a large surface area, which in turn reduces the surface area for the exciter electrodes and implies an increase in the excitation voltage. The auxiliary electrode must also have an additional output in the form of a thin wire. When the movable element is moving at high speeds this reduces the operational safety of the motor.
In addition, for example from U.S. Pat. No. 5,872,418, a method and a device for exciting an ultrasonic motor are known, in which the frequency of the electrical excitation voltage of the ultrasonic actuator is regulated by a constant phase difference being maintained between the voltage exciting the actuator and the current flowing through said actuator. In this method and the corresponding device a sinusoidal electrical voltage which stimulates the ultrasonic actuator is applied to the piezo-element of the ultrasonic actuator. The applied sinusoidal voltage causes a sinusoidal current to flow through the piezo-element.
The disadvantage of this method is also the fact that the phase difference between the electrical excitation voltage and the current flowing through the piezo-element depends on the mechanical load on the ultrasonic actuator. This is due to the fact that the sinusoidal current flowing through the piezo-element has two components, namely a capacitive current which flows through the electrical capacitance of the piezo-element, and a piezoelectric current which is determined by the angle of rotation of the domains of the piezo-element. At the mechanical resonant frequency, the piezoelectric current represents a so-called active current, or effective current. When there is a small mechanical load on the ultrasonic actuator, the active resistance of the actuator is significantly smaller than its reactive resistance. Therefore, the phase shift between the excitation voltage and the current flowing through the piezo-element at the mechanical resonant frequency is small and approaches zero. When the mechanical load applied to the actuator increases, the active resistance is increased, while the reactive resistance remains constant. The increase in the mechanical load therefore causes an increase in the phase angle between the excitation voltage and the current flowing through the piezo-element of the ultrasonic actuator.